Wallace, Robert M.

Permanent URI for this collectionhttps://hdl.handle.net/10735.1/2335

Robert M. Wallace is the Erik Jonsson Distinguished Chair and Professor of Materials Science and Engineering. In 2003, he joined the faculty in the Erik Jonsson School of Engineering and Computer Science at the University of Texas at Dallas (UTD) as a Professor of Electrical Engineering and Physics. He is a founding member of the Materials Science and Engineering program at UTD, served as an interim head for the program, and facilitated the transformation of the program into a department. In addition to his research program in nanoelectronic materials, Wallace was also the director of the new 5000 sq. ft. Cleanroom Research Laboratory for 6 years, supervising a staff of 9 and an annual budget >$2M. He was responsible for the oversight of the facility construction, tool purchases and installation, and assembly of the staff for the facility. Dr. Wallace also has courtesy appointments in the Departments of Electrical Engineering, Mechanical Engineering, and Physics at UT-Dallas. In 2018 Clarivate Analytics added him to their list of "Highly Cited Researchers." He has continued to be on that list through 2020.

ORCID page


2020 recipient of the Web of Science Highly Cited Researchers award for Engineering,Materials Science, and Physics


Recent Submissions

Now showing 1 - 20 of 31
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    Using Photoelectron Spectroscopy in the Integration of 2D Materials for Advanced Devices
    (Elsevier Science B.V., 2019-01-31) Addou, Rafik; Wallace, Robert M.; 0000-0001-5566-4806 (Wallace, RM); Addou, Rafik; Wallace, Robert M.
    The first commercial applications of two dimensional (2D) layered materials such as graphite and MoS₂ used their lubricant properties. Following the discovery of graphene and its potential applications in various fields, increased interest has focused on other 2D materials such as transition metal dichalcogenides (TMDs) offering tremendous opportunities in advanced optoelectronics and ultra-thin electronics. Using X-ray photoelectron spectroscopy (XPS), this review addresses the facets of the device fabrication and integration and correlates at the nanometer scale the device behavior to the TMD properties. Understanding and solving the integration challenges will make the TMD technology jump from the current phase of experimental proof of concept and laboratory research to a relevant prototype demonstration and production phase.
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    Engineering The Palladium-WSe₂ Interface Chemistry for Field Effect Transistors with High-Performance Hole Contacts
    (Amer Chemical Soc, 2018-12-07) Smyth, Christopher M.; Walsh, Lee A.; Bolshakov, Pavel; Catalano, Massimo; Addou, Rafik; Wang, Luhua; Kim, Jiyoung; Kim, Moon J.; Young, Chadwin D.; Hinkle, Christopher L.; Wallace, Robert M.; 0000-0001-5566-4806 (Wallace, RM); 0000-0003-0690-7423 (Young, CD); 0000-0003-2781-5149 (Kim, J); 0000-0002-6688-8626 (Walsh, LA); 0000-0002-5485-6600 (Hinkle, CD); 0000-0002-5454-0315 (Addou, R); 70133685 (Kim, J); Smyth, Christopher M.; Walsh, Lee A.; Bolshakov, Pavel; Catalano, Massimo; Addou, Rafik; Wang, Luhua; Kim, Jiyoung; Kim, Moon J.; Young, Chadwin D.; Hinkle, Christopher L.; Wallace, Robert M.
    Palladium has been widely employed as a hole contact to WSe₂ and has enabled, at times, the highest WSe₂ transistor performance. However, there are orders of magnitude variation across the literature in Pd-WSe₂ contact resistance and I-ON/I-OFF ratios with no true understanding of how to consistently achieve high-performance contacts. In this work, WSe₂ transistors with impressive I-ON/I-OFF ratios of 10(6) and Pd-WSe₂ Schottky diodes with near-zero variability are demonstrated utilizing Ohmic-like Pd contacts through deliberate control of the interface chemistry. The increased concentration of a PdSeₓ intermetallic is correlated with an Ohmic band alignment and concomitant defect passivation, which further reduces the contact resistance, variability, and barrier height inhomogeneity. The lowest contact resistance occurs when a 60 min post-metallization anneal at 400 degrees C in forming gas (FG) is performed. X-ray photoelectron spectroscopy indicates this FG anneal produces 3x the concentration of PdSeₓ and an Ohmic band alignment, in contrast to that detected after annealing in ultrahigh vacuum, during which a 0.2 eV hole Schottky barrier forms. Raman spectroscopy and scanning transmission electron microscopy highlight the necessity of the fabrication step to achieve high-performance contacts as no PdSeₓ forms, and WSe₂ is unperturbed by room temperature Pd deposition. However, at least one WSe₂ layer is consumed by the necessary interface reactions that form PdSeₓ requiring strategic exploitation of a sacrificial WSe₂ layer during device fabrication. The interface chemistry and structural properties are correlated with Pd-WSe₂ diode and transistor performance, and the recommended processing steps are provided to enable reliable high-performance contact formation.
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    UV-Ozone Functionalization of 2D Materials
    (Springer, 2018-10-04) McDonnell, Stephen J.; Wallace, Robert M.; 0000-0001-5566-4806 (Wallace, RM); Wallace, Robert M.
    Integrating two-dimensional (2D) materials into the current nanoelectronic process requires control over the deposition of gate oxides onto these materials. Atomic layer deposition (ALD) relies on surface dangling bonds that are scarce for 2D materials. This review summarizes the advances made in understanding and controlling the nucleation of ALD oxides on these materials. As an example, we focus on ozone-based processes including UV-ozone pretreatments, which we have found to effectively functionalize the surface of molybdenum disulfide. Furthermore, we discuss the advantages and limitations of various functionalization or seeding techniques, such as limits in scalability or damage to the 2D materials.
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    Enhancing Interconnect Reliability and Performance by Converting Tantalum to 2D Layered Tantalum Sulfide at Low Temperature
    (Wiley-VCH Verlag, 2019-06-11) Lo, C. -L; Catalano, Massimo; Khosravi, Ava; Ge, W.; Ji, Y.; Zemlyanov, D. Y.; Wang, Luhua; Addou, Rafik; Liu, Y.; Wallace, Robert M.; Kim, Moon J.; Chen, Z.; 0000-0001-5566-4806 (Wallace, RM); Catalano, Massimo; Khosravi, Ava; Wang, Luhua; Addou, Rafik; Wallace, Robert M.; Kim, Moon J.
    The interconnect half-pitch size will reach ≈20 nm in the coming sub-5 nm technology node. Meanwhile, the TaN/Ta (barrier/liner) bilayer stack has to be >4 nm to ensure acceptable liner and diffusion barrier properties. Since TaN/Ta occupy a significant portion of the interconnect cross-section and they are much more resistive than Cu, the effective conductance of an ultrascaled interconnect will be compromised by the thick bilayer. Therefore, 2D layered materials have been explored as diffusion barrier alternatives. However, many of the proposed 2D barriers are prepared at too high temperatures to be compatible with the back-end-of-line (BEOL) technology. In addition, as important as the diffusion barrier properties, the liner properties of 2D materials must be evaluated, which has not yet been pursued. Here, a 2D layered tantalum sulfide (TaSₓ) with ≈1.5 nm thickness is developed to replace the conventional TaN/Ta bilayer. The TaSx ultrathin film is industry-friendly, BEOL-compatible, and can be directly prepared on dielectrics. The results show superior barrier/liner properties of TaSₓ compared to the TaN/Ta bilayer. This single-stack material, serving as both a liner and a barrier, will enable continued scaling of interconnects beyond 5 nm node. ©2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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    High-κ Dielectric on ReS₂: In-Situ Thermal Versus Plasma-Enhanced Atomic Layer Deposition of Al₂O₃
    (MDPI AG, 2019-03-30) Khosravi, Ava; Addou, Rafik; Catalano, Massimo; Kim, Jiyoung; Wallace, Robert M.; 0000-0003-2781-5149 (Kim, J); 0000-0001-5566-4806 (Wallace, RM); 0000-0001-9901-9809 (Khosravi, A); 0000-0002-5454-0315 (Addou, R); 70133685 (Kim, J); Khosravi, Ava; Addou, Rafik; Catalano, Massimo; Kim, Jiyoung; Wallace, Robert M.
    We report an excellent growth behavior of a high-κ dielectric on ReS₂ , a two-dimensional (2D) transition metal dichalcogenide (TMD). The atomic layer deposition (ALD) of an Al₂O₃ thin film on the UV-Ozone pretreated surface of ReS₂ yields a pinhole free and conformal growth. In-situ half-cycle X-ray photoelectron spectroscopy (XPS) was used to monitor the interfacial chemistry and ex-situ atomic force microscopy (AFM) was used to evaluate the surface morphology. A significant enhancement in the uniformity of the Al₂O₃ thin film was deposited via plasma-enhanced atomic layer deposition (PEALD), while pinhole free Al₂O₃ was achieved using a UV-Ozone pretreatment. The ReS₂ substrate stays intact during all different experiments and processes without any formation of the Re oxide. This work demonstrates that a combination of the ALD process and the formation of weak S-O bonds presents an effective route for a uniform and conformal high-κ dielectric for advanced devices based on 2D materials. © 2019 by the authors.
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    Sensitivity of High-k Encapsulated MoS₂ Transistors to I-V Measurement Execution Time
    (Institute of Electrical and Electronics Engineers Inc.) Bolshakov, Pavel; Khosravi, Ava; Zhao, Peng; Wallace, Robert M.; Young, Chadwin D.; Hurley, P. K.; Bolshakov, Pavel; Khosravi, Ava; Zhao, Peng; Wallace, Robert M.; Young, Chadwin D.
    High-k encapsulated MoS₂ field-effect-transistors were fabricated and electrically characterized. Comparison between HfO₂ and Al₂O₃ encapsulated MoS₂ FETs and their I-V response to execution time are shown. Changes in gate voltage step and integration time demonstrate that electrical characterization parameters can significantly impact device parameters such as the subthreshold swing and the threshold voltage. © 2018 IEEE.
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    Dual-Gate MoS₂ Transistors with Sub-10 NM Top-Gate High-K Dielectrics
    (American Institute of Physics Inc.) Bolshakov, Pavel; Khosravi, Ava; Zhao, Peng; Hurley, P. K.; Hinkle, Christopher L.; Wallace, Robert M.; Young, Chadwin D.; 0000-0002-3530-6400 (Zhao, P); 0000-0001-5566-4806 (Wallace, RM); 0000-0003-0690-7423 (Young, CD); Bolshakov, Pavel; Khosravi, Ava; Zhao, Peng; Hinkle, Christopher L.; Wallace, Robert M.; Young, Chadwin D.
    High quality sub-10 nm high-k dielectrics are deposited on top of MoS₂ and evaluated using a dual-gate field effect transistor configuration. Comparison between top-gate HfO₂ and an Al₂O₃/HfO₂ bilayer shows significant improvement in device performance due to the insertion of the thin Al₂O₃ layer. The results show that the Al₂O₃ buffer layer improves the interface quality by effectively reducing the net fixed positive oxide charge at the top-gate MoS₂/high-k dielectric interface. Dual-gate sweeping, where both the top-gate and the back-gate are swept simultaneously, provides significant insight into the role of these oxide charges and improves overall device performance. Dual-gate transistors encapsulated in an Al₂O₃ dielectric demonstrate a near-ideal subthreshold swing of ~60 mV/dec and a high field effect mobility of 100 cm²/V·s.
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    Covalent Nitrogen Doping in Molecular Beam Epitaxy-Grown and Bulk WSe₂
    (American Institute of Physics Inc, 2018-10-22) Khosravi, Ava; Addou, Rafik; Smyth, Christopher M.; Yue, Ruoyu; Cormier, Christopher R.; Kim, Jiyoung; Hinkle, Christopher L.; Wallace, Robert M.; 0000-0002-5454-0315 (Addou, R); 0000-0003-2781-5149 (Kim, J); 0000-0001-5566-4806 (Wallace, RM); 70133685 (Kim, J); Addou, Rafik; Smyth, Christopher M.; Yue, Ruoyu; Cormier, Christopher R.; Kim, Jiyoung; Hinkle, Christopher L.; Wallace, Robert M.
    Covalent p-type doping of WSe₂ thin films grown by molecular beam epitaxy and WSe₂ exfoliated from bulk crystals is achieved via remote nitrogen plasma exposure. X-ray photoelectron and Raman spectroscopies indicate covalently bonded nitrogen in the WSe₂ lattice as well as tunable nitrogen concentration with N₂ plasma exposure time. Furthermore, nitrogen incorporation induces compressive strain on the WSe₂ lattice after N₂ plasma exposure. Finally, atomic force microscopy and scanning tunneling microscopy reveal that N₂ plasma treatment needs to be carefully tuned to avoid any unwanted strain or surface damage.
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    Nucleation and Growth of WSe₂: Enabling Large Grain Transition Metal Dichalcogenides
    (IOP Publishing Ltd, 2017-09-22) Yue, Ruoyu; Nie, Yifan; Walsh, Lee A.; Addou, Rafik; Liang, Chaoping; Lu, Ning; Barton, Adam T.; Zhu, Hui; Che, Zifan; Barrera, Diego; Cheng, Lanxia; Cha, Pil-Ryung; Chabal, Yves J.; Hsu, Julia W. P.; Kim, Jiyoung; Kim, Moon J.; Colombo, Luigi; Wallace, Robert M.; Cho, Kyeongjae; Hinkle, Christopher L.; 0000-0002-2910-2938 (Liang, C); Yue, Ruoyu; Nie, Yifan; Walsh, Lee A.; Addou, Rafik; Liang, Chaoping; Lu, Ning; Barton, Adam T.; Zhu, Hui; Che, Zifan; Barrera, Diego; Cheng, Lanxia; Chabal, Yves J.; Hsu, Julia W. P.; Kim, Jiyoung; Kim, Moon J.; Wallace, Robert M.; Cho, Kyeongjae; Hinkle, Christopher L.
    The limited grain size (< 200 nm) for transition metal dichalcogenides (TMDs) grown by molecular beam epitaxy (MBE) reported in the literature thus far is unsuitable for high-performance device applications. In this work, the fundamental nucleation and growth behavior of WSe₂ is investigated through a detailed experimental design combined with on-lattice, diffusion-based first principles kinetic modeling to enable large area TMD growth. A three-stage adsorption-diffusion-attachment mechanism is identified and the adatom stage is revealed to play a significant role in the nucleation behavior. To limit the nucleation density and promote 2D layered growth, it is necessary to have a low metal flux in conjunction with an elevated substrate temperature. At the same time, providing a Se-rich environment further limits the formation of W-rich nuclei which suppresses vertical growth and promotes 2D growth. The fundamental understanding gained through this investigation has enabled an increase of over one order of magnitude in grain size for WSe₂ thus far, and provides valuable insight into improving the growth of other TMD compounds by MBE and other growth techniques such as chemical vapor deposition (CVD).
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    A Crystalline Oxide Passivation on In₀․₅₃Ga₀․₄₇As (100)
    (American Institute of Physics Inc, 2018-09-24) Qin, Xiaoye; Wang, W. -E; Droopad, R.; Rodder, M. S.; Wallace, Robert M.; 0000-0001-5566-4806 (Wallace, RM); Qin, Xiaoye; Wallace, Robert M.
    The passivation of In₀․₅₃Ga₀․₄₇As surfaces is highly desired for transistor performance. In this study, the feasibility of a crystalline oxide passivation on In₀․₅₃Ga₀․₄₇As (100) is demonstrated experimentally. The (3 × 1) and (3 × 2) crystalline oxide reconstructions are formed on the de-capped In₀․₅₃Ga₀․₄₇As (100) surfaces through the control of the surface oxidation states. By monitoring the evolution of chemical states and associated structures of the In₀․₅₃Ga₀․₄₇As (100) surfaces upon O₂ and subsequent atomic hydrogen exposure, we find that the control of the Ga oxide states is critical to the formation of the crystalline oxide reconstructions. The stability of the crystalline oxide layers upon the atomic layer deposition of HfO₂ is investigated as well. Furthermore, the capacitance voltage behavior of metal oxide semiconductor capacitors with an HfO₂ dielectric layer reveals that the crystalline oxide reconstructions result in a decrease in the density of interface traps (D_{it}) from ∼1 × 10¹³ cm⁻² eV⁻¹ to ∼1 × 10¹² cm⁻² eV⁻¹ compared with the de-capped surface. The crystalline oxide passivation offers a platform to develop In₀․₅₃Ga₀․₄₇As devices with a low density of interface states.
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    A Kinetic Monte Carlo Simulation Method of Van Der Waals Epitaxy for Atomistic Nucleation-Growth Processes of Transition Metal Dichalcogenides
    (Nature Publishing Group, 2018-08-31) Nie, Yifan; Liang, Chaoping; Cha, Pil-Ryung; Colombo, Luigi; Wallace, Robert M.; Cho, Kyeongjae; 0000-0003-4771-3633 (Nie, Y); Nie, Yifan; Liang, Chaoping; Cha, Pil-Ryung; Wallace, Robert M.; Cho, Kyeongjae
    Controlled growth of crystalline solids is critical for device applications, and atomistic modeling methods have been developed for bulk crystalline solids. Kinetic Monte Carlo (KMC) simulation method provides detailed atomic scale processes during a solid growth over realistic time scales, but its application to the growth modeling of van der Waals (vdW) heterostructures has not yet been developed. Specifically, the growth of single-layered transition metal dichalcogenides (TMDs) is currently facing tremendous challenges, and a detailed understanding based on KMC simulations would provide critical guidance to enable controlled growth of vdW heterostructures. In this work, a KMC simulation method is developed for the growth modeling on the vdW epitaxy of TMDs. The KMC method has introduced full material parameters for TMDs in bottom-up synthesis: metal and chalcogen adsorption/desorption/diffusion on substrate and grown TMD surface, TMD stacking sequence, chalcogen/metal ratio, flake edge diffusion and vacancy diffusion. The KMC processes result in multiple kinetic behaviors associated with various growth behaviors observed in experiments. Different phenomena observed during vdW epitaxy process are analysed in terms of complex competitions among multiple kinetic processes. The KMC method is used in the investigation and prediction of growth mechanisms, which provide qualitative suggestions to guide experimental study.
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    Al₂O₃ on WSe₂ by Ozone Based Atomic Layer Deposition: Nucleation and Interface Study
    Azcatl, Angelica; Wang, Qingxiao; Kim, Moon J.; Wallace, Robert M.; 0000-0001-5566-4806 (Wallace, RM); Azcatl, Angelica; Wang, Qingxiao; Kim, Moon J.; Wallace, Robert M.
    In this work, the atomic layer deposition process using ozone and trimethylaluminum (TMA) for the deposition of Al₂O₃ films on WSe₂ was investigated. It was found that the ozone-based atomic layer deposition enhanced the nucleation of Al₂O₃ in comparison to the water/TMA process. In addition, the chemistry at the Al₂O₃ / WSe₂ interface and the surface morphology of the Al₂O₃ films exhibited a dependence on the deposition temperature. A non-covalent functionalizing effect of ozone on WSe₂ at low deposition temperatures 30 ⁰C was identified which prevented the formation of pinholes in the Al₂O₃ films. These findings aim to provide an approach to obtain high-quality gate dielectrics on WSe₂ for two-dimensional transistor applications.
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    Tuning Electronic Transport in Epitaxial Graphene-Based Van Der Waals Heterostructures
    (RSC Pub) Lin, Yu-Chuan; Li, Jun; de la Barrera, Sergio,C.; Eichfeld, Sarah M.; Nie, Yifan; Addou, Rafik; Mende, Patrick C.; Wallace, Robert M.; Cho, Kyeongjae; Feenstra, Randall M.; Robinson, Joshua A.; 0000-0001-5566-4806 (Wallace, RM); 0000-0003-2698-7774 (Cho, K); Nie, Yifan; Addou, Rafik; Wallace, Robert M.; Cho, Kyeongjae
    Two-dimensional tungsten diselenide (WSe₂) has been used as a component in atomically thin photovoltaic devices, field effect transistors, and tunneling diodes in tandem with graphene. In some applications it is necessary to achieve efficient charge transport across the interface of layered WSe₂-graphene, a semiconductor to semimetal junction with a van der Waals (vdW) gap. In such cases, band alignment engineering is required to ensure a low-resistance, ohmic contact. In this work, we investigate the impact of graphene electronic properties on the transport at the WSe₂-graphene interface. Electrical transport measurements reveal a lower resistance between WSe₂ and fully hydrogenated epitaxial graphene (EGFH) compared to WSe₂ grown on partially hydrogenated epitaxial graphene (EGPH). Using low-energy electron microscopy and reflectivity on these samples, we extract the work function difference between the WSe₂ and graphene and employ a charge transfer model to determine the WSe₂ carrier density in both cases. The results indicate that WSe₂-EGFH displays ohmic behavior at small biases due to a large hole density in the WSe₂, whereas WSe₂-EGPH forms a Schottky barrier junction.;
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    Oxidation of GaSb(100) and its Control Studied by Scanning Tunneling Microscopy and Spectroscopy
    Makela, J.; Tuominen, M.; Yasir, M.; Kuzmin, M.; Dahl, J.; Punkkinen, M. P. J.; Laukkanen, P.; Kokko, K.; Wallace, Robert M.; 0000-0001-5566-4806 (Wallace, RM); Wallace, Robert M.
    Atomic-scale knowledge and control of oxidation of GaSb(100), which is a potential interface for energy-efficient transistors, are still incomplete, largely due to an amorphous structure of GaSb(100) oxides. We elucidate these issues with scanning-tunneling microscopy and spectroscopy. The unveiled oxidation-induced building blocks cause defect states above Fermi level around the conduction-band edge. By interconnecting the results to previous photoemission findings, we suggest that the oxidation starts with substituting second-layer Sb sites by oxygen. Adding small amount of indium on GaSb(100), resulting in a (4 x 2)-In reconstruction, before oxidation produces a previously unreported, crystalline oxidized layer of (1 x 3)-O free of gap states.
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    In Situ Plasma Enhanced Atomic Layer Deposition Half Cycle Study of Al₂O₃ on AlGaN/GaN High Electron Mobility Transistors
    (American Institute of Physics Inc.) Qin, Xiaoye; Wallace, Robert M.
    A half cycle study of plasma enhanced atomic layer deposited (PEALD) Al₂O₃ on AlGaN is investigated using in situ X-ray photoelectron spectroscopy, low energy ion scattering, and ex situ electrical characterizations. A faster nucleation or growth is detected from PEALD relative to purely thermal ALD using an H₂O precursor. The remote O₂ plasma oxidizes the AlGaN surface slightly at the initial stage, which passivates the surface and reduces the OFF-state leakage. This work demonstrates that PEALD is a useful strategy for Al₂O₃ growth on AlGaN/GaN devices. © 2015 AIP Publishing LLC.
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    Low Temperature Synthesis of Graphite on Ni Films Using Inductively Coupled Plasma Enhanced CVD
    (Royal Soc Chemistry) Cheng, Lanxia; Yun, Kayoung; Lucero, Antonio; Huang, Jie; Meng, Xin; Lian, Guoda; Nam, Ho-Seok; Wallace, Robert M.; Kim, Moon J.; Venugopal, Archana; Colombo, Luigi; Kim, Jiyoung; A-5283-2008 (Wallace, RM); A-2297-2010 (Kim, MJ); 70133685 (Kim, J)
    Controlled synthesis of graphite at low temperatures is a desirable process for a number of applications. Here, we present a study on the growth of thin graphite films on polycrystalline Ni films at low temperatures, about 380 ⁰C, using inductively coupled plasma enhanced chemical vapor deposition. Raman analysis shows that the grown graphite films are of good quality as determined by a low I-D/I-G ratio, ~0.43, for thicknesses ranging from a few layers of graphene to several nanometer thick graphitic films. The growth of graphite films was also studied as a function of time, precursor gas pressure, hydrogen concentration, substrate temperature and plasma power. We found that graphitic films can be synthesized on polycrystalline thin Ni films on SiO₂/Si substrates after only 10 seconds at a substrate temperature as low as 200 ⁰C. The amount of hydrogen radicals, adjusted by changing the hydrogen to methane gas ratio and pressure, was found to dramatically affect the quality of graphite films due to their dual role as a catalyst and an etchant. We also find that a plasma power of about 50 W is preferred in order to minimize plasma induced graphite degradation.
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    Atomically Thin Resonant Tunnel Diodes Built from Synthetic van der Waals Heterostructures
    (Nature Pub. Group) Lin, Yu-Chuan; Ghosh, Ram Krishna; Addou, Rafik; Lu, Ning; Eichfeld, Sarah M.; Zhu, Hui; Li, Ming-Yang; Peng, Xin; Kim, Moon J.; Li, Lain-Jong; Wallace, Robert M.; Datta, Suman; Robinson, Joshua A.; A-5283-2008 (Wallace, RM); A-2297-2010 (Kim, MJ)
    Vertical integration of two-dimensional van der Waals materials is predicted to lead to novel electronic and optical properties not found in the constituent layers. Here, we present the direct synthesis of two unique, atomically thin, multi-junction heterostructures by combining graphene with the monolayer transition-metal dichalcogenides: molybdenum disulfide (MoS₂), molybdenum diselenide (MoSe₂) and tungsten diselenide (WSe₂). The realization of MoS₂-WSe₂-graphene and WSe₂-MoS₂-graphene heterostructures leads to resonant tunnelling in an atomically thin stack with spectrally narrow, room temperature negative differential resistance characteristics.;
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    High Quality HfO₂/p-GaSb(001) Metal-Oxide-Semiconductor Capacitors with 0.8 Nm Equivalent Oxide Thickness
    Barth, Michael; Rayner, G. Bruce,Jr.; McDonnell, Stephen; Wallace, Robert M.; Bennett, Brian R.; Engel-Herbert, Roman; Datta, Suman
    We investigate in-situ cleaning of GaSb surfaces and its effect on the electrical performance of p-type GaSb metal-oxide-semiconductor capacitor (MOSCAP) using a remote hydrogen plasma. Ultrathin HfO₂ films grown by atomic layer deposition were used as a high permittivity gate dielectric. Compared to conventional ex-situ chemical cleaning methods, the in-situ GaSb surface treatment resulted in a drastic improvement in the impedance characteristics of the MOSCAPs, directly evidencing a much lower interface trap density and enhanced Fermi level movement efficiency. We demonstrate that by using a combination of ex-situ and in-situ surface cleaning steps, aggressively scaled HfO₂/p-GaSb MOSCAP structures with a low equivalent oxide thickness of 0.8 nm and efficient gate modulation of the surface potential are achieved, allowing to push the Fermi level far away from the valence band edge high up into the band gap of GaSb. (c) 2014 AIP Publishing LLC.
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    A Crystalline Oxide Passivation for Al₂O₃/AlGaN/GaN
    (American Institute of Physics Inc.) Qin, Xiaoye; Dong, Hong; Kim, Jiyoung; Wallace, Robert M.; A-5283-2008 (Wallace, RM)
    In situ X-ray photoelectron spectroscopy and low energy electron diffraction are performed to study the formation of a crystalline oxide on the AlGaN surface. The oxidation of the AlGaN surface is prepared by annealing and remote N₂+O₂ plasma pretreatments resulting in a stable crystalline oxide. The impact of the oxide on the interface state density is studied by capacitance voltage (C-V) measurements. It is found that a remote plasma exposure at 550⁰ C shows the smallest frequency dispersion. Crystalline oxide formation may provide a novel passivation method for high quality AlGaN/GaN devices.
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    Accumulation Capacitance Frequency Dispersion of Ⅲ-Ⅴ Metal-Insulator-Semiconductor Devices due to Disorder Induced Gap States
    (American Institute of Physics Inc., 2014-07-07) Galatage, R. V.; Zhernokletov, Dmitry M.; Dong, Hong; Brennan, Barry; Hinkle, Christopher L.; Wallace, Robert M.; Vogel, E. M.
    The origin of the anomalous frequency dispersion in accumulation capacitance of metal-insulator-semiconductor devices on InGaAs and InP substrates is investigated using modeling, electrical characterization, and chemical characterization. A comparison of the border trap model and the disorder induced gap state model for frequency dispersion is performed. The fitting of both models to experimental data indicate that the defects responsible for the measured dispersion are within approximately 0.8nm of the surface of the crystalline semiconductor. The correlation between the spectroscopically detected bonding states at the dielectric/III-V interface, the interfacial defect density determined using capacitance-voltage, and modeled capacitance-voltage response strongly suggests that these defects are associated with the disruption of the III-V atomic bonding and not border traps associated with bonding defects within the high-k dielectric.

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